Wind-powered electric generators convert the movement of air into electricity by using the air movement to rotate aerodynamically shaped blades oriented to the airstream. The blades are mounted through a gearing system to an electrical generator which produces electric power. Although some models operate at constant speed, more energy can be collected by variable-speed turbines which use a solid-state power converter to interface to the transmission system.
Most wind turbines are horizontally oriented. In order to extract a maximum amount of power at any given airspeed, i.e., the speed with which the air is moving past a stationary point on the wind generator, the blades and their orientation to the airstream must be constantly reconfigured to cause the relative wind to encounter the blade at a predetermined optimal angle of attack. This is normally done by changing the pitch of the blade as the air flow increases or decreases, or as the direction from which it is coming varies. The blades, typically three to a wind turbine, are attached to a rotating hub which is centered at an end of a nacelle. The nacelle may include gearing and clutch components configured to drive an electrical generator at a more or less constant speed regardless of the wind speed or rotational angular velocity of the hub.
The method of varying blade pitch is particularly advantageous in capturing wind power during operation under light to medium air flow conditions. However, occasionally the wind speed may be so high as to exceed the design capacity of the turbine or associated electric generator. Under such conditions, it may be desirable to vary the blade pitch to cause the blade to rotate at slower angular velocities than the theoretical maximum that could be reached. In the event of a mechanical failure, it may be necessary to slow or fully stop rotation of the blade. One way of slowing or stopping the rotation of a blade is to vary the pitch of the blade such that air passing over the blade ceases to create lift. This may be accomplished either by positioning the blade at an angle to the relative wind as to “feather” the blade (an angle at which the forces generated at either side of the blade neutralize one another), or by positioning the blade at such an extreme angle to the relative wind as to cause the blade to stall.
The blades attached to the wind turbine typically have lengths ranging between 30-40 meters, and rotate at 10-22 revolutions per minute, developing speeds at the blade tips of up to 91 m/s. The force that a moving air mass exerts upon such blades can be considerable, and the control mechanisms needed to hold a blade at a given angle during operation must be rugged and reliable. Hydraulic actuators are normally used to vary the pitch of a wind turbine blade. The actuators are situated within the rotating hub having one end affixed to a hard point on the hub and the other being affixed to a point on the blade whereby extension or compression of the actuator will cause the blade to rotate about its longitudinal axis, thereby varying the angle of attack with the relative wind. Being mounted in the hub of the turbine, the actuators are subjected to extreme forces and vibration generated as the hub is turning. When used for pitch control, the actuators are working virtually all the time the hub is turning, adjusting the blades numerous times every minute. The same actuators are used to supply power for an emergency stop.
During normal operation within a normal operating range, varying the pitch of a blade will be sufficient to maintain its rotational speed within design limits. However, if conditions should develop in which normal design limits are being exceeded, e.g., extraordinary wind velocities caused by microbursts or downdrafts associated with extreme weather conditions; or mechanical failure of a turbine shaft or gearing mechanism; or the dropping of electrical load through a broken line or other electrical anomaly, then the normal electrical or mechanical load being carried by the wind turbine may be lost, and the blade may start to spin out of control. Such a condition, if allowed to develop, has the potential to cause catastrophic failure of the entire wind turbine and its associated mechanical supporting structure. To prevent this kind of catastrophic failure, the same hydraulic actuators that control the pitch of turbine blades during normal operation, are also used to automatically move a blade to a predetermined angle upon the loss of hydraulic power or control mechanisms.
Hydraulic systems used in wind turbine generators typically operate at pressures about 180 to 200 bar (about 2600-3000 psi). System pressure is maintained with electrically driven hydraulic pumps. However, in the event of hydraulic failure, such as the failure of a pump, residual hydraulic power may be temporarily stored for a limited time in hydraulic accumulators integral to the system. Hydraulic accumulators are well known, and operate by storing a compressed gas, usually nitrogen, within a non-gas-permeable bladder surrounded by a volume of hydraulic fluid, all of which is contained within a rigid canister. During installation or maintenance, the bladder will first be filled with gas to a pressure of 110 bar (about 1600 psi). Then, hydraulic fluid will be introduced into the canister and brought up to system pressure. Since the system pressure is greater than the initial bladder pressure, the gas in the bladder will compress until, according to Boyle's law, the decrease in the volume of the gas has caused the pressure within the bladder to rise until it is the same as the hydraulic system pressure. The bladder is compressed from all directions, and may take on the shape of a wrinkled raisin, or possibly a starburst.
The compressed gas acts as a damper to shocks that may occur in the hydraulic system during normal operation; and, should a hydraulic pump fail and the system lose pressure, the pressurized gas will expand within the bladder, forcing hydraulic fluid out of the canister and into the system to create sufficient hydraulic pressure to activate emergency systems, such as shutdown pitch control actuators. Because hydraulic actuators are used in hydraulic systems that control blade pitch, they are located in the hub, and are subject to the rotational forces and related consequences present within a rotating hub.
A known problem with emergency shutdown systems is that the flexible non-gas-permeable bladders used in hydraulic accumulators have a relatively short lifespan with respect to other components used in wind turbine electrical generators. If a bladder should be breached, that failure results in a potentially unsafe condition—loss of emergency hydraulic power—that could allow a non-critical failure of a hydraulic pump to rapidly escalate into a catastrophic failure that could destroy an entire unit. Thus, whenever the bladder in an accumulator fails, the unit must be shut down and the hub disassembled sufficiently that the accumulator can be replaced. Such shutdowns are wasteful, costly, and inefficient. Accordingly, what is needed is a system whereby the service life of a non-gas-permeable bladder in a hydraulic accumulator may be significantly extended without compromising the safety or integrity of the unit in which it is installed.